The invention relates to magnetic thin film media and methods for their fabrication and more particularly to magnetic thin film disks having a protective layer structure composed of carbon and nitrogen (CNx) and wherein the of the protective layer has more one composition of CNx.
A typical prior art head and disk system 10 is illustrated in FIG. 1. In operation the magnetic transducer 20 is supported by the suspension 13 as it flies above the disk 16. The magnetic transducer 20, usually called a xe2x80x9cheadxe2x80x9d or xe2x80x9cslider,xe2x80x9d is composed of elements that perform the task of writing magnetic transitions (the write head 23) and reading the magnetic transitions (the read head 12). The electrical signals to and from the read and write heads 12, 23 travel along conductive paths (leads) 14 which are attached to or embedded in the suspension 13. The magnetic transducer 20 is positioned over points at varying radial distances from the center of the disk 16 to read and write circular tracks (not shown). The disk 16 is attached to a spindle 18 that is driven by a spindle motor 24 to rotate the disk 16. The disk 16 comprises a substrate 26 on which a plurality of thin films 21 are deposited. The thin films 21 include ferromagnetic material in which the write head 23 records the magnetic transitions in which information is encoded. The protective layer structure (not shown) is typically the last or uppermost layer.
The conventional disk 16 typically has a substrate 26 of AlMg or glass. The thin films 21 on the disk 16 typically include a chromium or chromium alloy underlayer that is deposited on the substrate 26. The magnetic layer in the thin films 21 is based on various alloys of cobalt, nickel and iron. For example, a commonly used alloy is CoPtCr. However, additional elements such as tantalum and boron are often used in the magnetic alloy.
The protective overcoat layer 35 is used to improve wearability and corrosion. The materials and/or compositions which are optimized for one performance characteristic are rarely optimized for others. The most commonly used protective layer materials for commercial thin film disks have been carbon, hydrogenated carbon (CHx), nitrogenated carbon (CNx) and CNxHy. Efforts to optimize overcoat properties have included use of a layer structure using different materials and/or compositions for each of two or more layers in the overcoat structure. For example, U.S. Pat. No. 5,942,317 issued to R. White describes the use of a graded CHx protective layer wherein the hydrogen content is highest at the film""s surface to take advantage of the lower polar surface energy characteristic of higher hydrogen levels (which improves corrosion resistance) and is lowest at the interface with the magnetic layer to optimize the adhesion properties. The midlevel of the CHx film is likewise optimized by having an intermediate hydrogen concentration which has a high hardness to improve wearability. The variations in the hydrogen can be continuous or discrete. For example, a protective layer structure with three sublayers with low hydrogen concentration nearest the magnetic layer, intermediate hydrogen concentration in the middle sublayer and high hydrogen concentration at the surface is suggested by White ""317.
In U.S. Pat. No. 5,679,431 Chen, et al., describe the use of a bilayer protective overcoat in which the initial sublayer is carbon, titanium or chromium and the surface sublayer is CHx or CNx. The problem being addressed in Chen ""431 is diffusion of nitrogen or hydrogen into the magnetic layer. The initial sublayer is intended to act as a diffusion barrier.
The applicants disclose a thin film structure comprising a protective layer structure having at least two different compositions of carbon and nitrogen with the surface of the protective layer having the lowest nitrogen content. The protective layer structure of the invention is preferably used over the magnetic layer on a thin film magnetic disk. The applicants have discovered that the durability of a relatively high nitrogen content CNx protective layer structure is improved by the addition of a surface sublayer of CNx with a relatively low nitrogen concentration. The resulting protective layer structure provides superior durability to either of the thin film compositions used alone; thus, a synergistic result is achieved. The protective layer structure of the invention has the additional benefit of decreasing the polar surface energy and therefore, improving the corrosion resistance of the film structure. In one embodiment the protective layer of the invention is implemented as two discrete sublayers with the surface sublayer having the lowest nitrogen concentration. In this embodiment the surface sublayer has from 0 to 8 at. % nitrogen and the higher nitrogen content is between 10 and 25 at. % nitrogen at the magnetic layer interface. The protective layer structure of the invention can also be implemented as a film in which the nitrogen content gradually decreases between the magnetic layer and the surface of the protective layer structure.